1. Field of the Invention
The present invention relates generally to integrated circuits and devices, and, more particularly to matching the threshold voltages of enhancement mode and depletion mode devices and reducing output capacitance of gallium nitride (GaN) devices.
2. Description of the Related Art
GaN semiconductor devices are increasingly desirable because of their ability to switch at high frequency, to carry large current, and to support high voltages. Development of these devices has generally been aimed at high power/high frequency applications. Devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFET), high electron mobility transistors (HEMT), or modulation doped field effect transistors (MODFET). These types of devices can typically withstand high voltages, e.g., 30V-to-2000 Volts, while operating at high frequencies, e.g., 100 kHZ-100 GHz.
A GaN HEMT device includes a nitride semiconductor with at least two nitride layers. Different materials formed on the semiconductor or on a buffer layer causes the layers to have different band gaps. The different material in the adjacent nitride layers also causes polarization, which contributes to a conductive two dimensional electron gas (2DEG) region near the junction of the two layers, specifically in the layer with the narrower band gap.
The nitride layers that cause polarization typically include a barrier layer of AlGaN adjacent to a layer of GaN to include the 2DEG, which allows charge to flow through the device. This barrier layer may be doped or undoped. Because the 2DEG region exists under the gate at zero gate bias, most nitride devices are normally on, or depletion mode devices. If the 2DEG region is depleted (i.e., removed) below the gate at zero applied gate bias, the device can be an enhancement mode device. Enhancement mode devices are normally off and are desirable because of the added safety they provide and because they are easier to control with simple, low cost drive circuits. An enhancement mode device requires a positive bias applied at the gate in order to conduct current.
In some integrated circuit designs, a high electron mobility transistor (HEMT) or pseudomorphic High-Electron Mobility Transistor ((p-)HEMT) is divided into a depletion mode transistor having a negative value of threshold voltage VTh and an enhancement mode transistor having a positive value of threshold voltage VTh. In such designs, it is desirable for the absolute value of the threshold voltages VTh of the enhancement mode and depletion mode devices to be equal. For example, if the enhancement mode threshold voltage VTh is positive 1.5 volts, the depletion mode device threshold voltage VTh should be negative 1.5 volts.
The present invention provides an approach to achieve enhancement mode and depletion mode devices with the same absolute value.